I would add excellent headphones have very little phase shift, as their single diaphragm moves nearly/almost as one solid piston through the range of 200Hz to >8kHz
In that same range, most all speakers have hundreds of degrees of phase shift (time delays), which change with the frequency being reproduced.
Roy,
Agreed. Minor and gradual (smooth) changes in phase seem to be extremely important in tranducer design between about 300 and 4 Khz (preserving timbre over the senstive mid range frequencies ).
I suspect this is one of the reasons speakers with similar frequency response sound so different. I suspect that another reason stems from differences in off-axis response or dispersion.
So two alike speakers in on axis frequency response can sound totally different. So why do the audio magazines and specs all look primarily at on axis freq response?....go figure!!!
Other factors are IM and harmonic distortion ( again something audio mags rarely delve into and are often horrific in many speakers at higher or realistic listening levels and especially at low frequencies but also, suprisingly, bang in the upper mids/treble where it is most audible (ringing, breakup)...again nobody seems to look at this....go figure!!!)
Further factors are baffle and edge diffraction.....again nobody examines or measures these effects as a test on commercial products....again ....go figure!
BTW: I still have my trusty AKG 240's (1979 vintage Stucio headphones), the first headphones that really blew me away. I think the Koss were stunning too - although I found them a bit heavy/uncomfortable to wear. (However, I do find headphone listening an unnatural sound even if distortion is more easily heard. It is not just the "in your head" sound but the lack of reverberation that I don't like... a certain reverberation of a live room is quite essential, IMHO, in hearing more detail in the music as the reverberations help seperate sounds and let the brain/ear gleen more - so I rarely use headphones as a good pair of loud speakers simply wins hands down!)